High accuracy is desired in speed and angle sensors. For speed sensors, a highly accurate sampling of the transducer wheel is desired in order to get an optimum angle resolution. Where pole wheels are the transducer wheel, an increase in the number of poles at a certain wheel diameter can support a resolution enhancement but at the expense of the magnetic field and, thereby, the working distance.
For speed sensors based on angle sensors, such as xMR sensors in a “top read” configuration in which the sensor is positioned on the face side of the pole wheel such that rotating field vectors are detected, frequency doubling can be used to overcome a compatibility problem between AMR and GMR/TMR sensors. AMR angle sensors exhibit inherent 180-degree uniqueness, while GMR and TMR sensors have 360-degree uniqueness, i.e. upon a rotating magnetic field a 360-degree rotation results in a single period of the output signal for the GMR/TMR angle sensor and a double period for the AMR angle sensor. With a frequency doubling of the GMR/TMR angle sensors, AMR sensors easily could be replaced by GMR/TMR sensors without changing the related signal evaluation circuitry. Both high accuracy and high resolution typically are desired for angle sensors, at least within a limited range.
Conventional solutions suffer from drawbacks. For example, some speed sensors use a digital pulse multiplication technique, which involves an incremental interpolation of the output signal on an external magnetic field. Disadvantages of such an approach are complex circuitry and that a full magnetic field period is needed to enable the interpolation and multiplication, which does not work for small angle ranges. Other magnetoresistive speed sensors use the inherent frequency doubling property of AMR angle sensors in a “top read” configuration in combination with magnetic pole wheels, which also present disadvantages with respect to smaller signal size.
Therefore, there is a need for improved speed and angle sensors.